Recent Advancement in Photocatalytic Desulphurization using Decorated Photocatalysts

A review on recent progression of photocatalytic desulphurization study over decorated photocatalysts

C.N.C. Hitam

^a

, A.A. Jalil

^a,b,

*, A.A. Abdulrasheed

^a,c

aSchoolofChemicalandEnergyEngineering,FacultyofEngineering,UniversitiTeknologiMalaysia,81310UTMJohorBahru,Johor,Malaysia

bCentreofHydrogenEnergy,InstituteofFutureEnergy,UniversitiTeknologiMalaysia,81310UTMJohorBahru,Johor,Malaysia

cDepartmentofChemicalEngineering,AbubakarTafawaBalewaUniversity,PMB0248Bauchi,BauchiState,Nigeria

ARTICLE INFO

Articlehistory:

Received26December2018

Receivedinrevisedform13February2019 Accepted25February2019

Availableonline5March2019

Keywords:

Recentprogression

Photocatalyticdesulphurization Decoratedphotocatalysts SOxemission

Eye-catchingtechnology

ABSTRACT

Photocatalyticoxidativedesulphurization(PODS)hasbecomeaneye-catchinggreentechnologyforfuel oiltreatmentbecauseofitsmildreactionconditions.Thepresentreviewconsistsoftheadvantagesand disadvantagesofPODScatalystsandthecharacteristicsofexcellentPODScatalysts.Explicitly,detailed strategiesforobtainingpromisingperformanceincludingtheuseofsupportmaterials,dopingofmetal elements,couplingwithothersuitablesemiconductors,andtheinfluenceofotherimportantcriteriain PODSaredeliberatedtofinalizeamechanisticstudyofanexcellentPODSmethod.Besides,thefuture prospectsandchallengesarealsoincludedtohighlighttheunexploredpotentialofPODScriteria.

©2019TheKoreanSocietyofIndustrialandEngineeringChemistry.PublishedbyElsevierB.V.Allrights reserved.

Introduction

Crude oilis comprised ofanextensive assortmentof hydro- carbons and other mixtures containing variable amounts of organosulphurcompounds (OSCs),nitrogen,andoxygen, which mostly remain in refined petroleum products including diesel, gasoline,andjetfuel[1,2].TheexistenceofOSCsinthosefuelsare uninvitedduetotheemissionofSOxfromtheircombustion,which is a main causeof global warming, acidrain, and atmospheric contamination, as well as several health problems including respiratoryillnesses,heartdisease,andasthma[3,4].Inautomo- tiveandrefiningprocesses,sulphurisalsoundesirableasittends todeactivatethecatalystsandcauses corrosionproblemsinthe pipelineandrefiningequipment[5].Infuel,sulphurcompounds existin differentforms that can becategorized intofour main groups which are mercaptans, sulphides, disulphides, and thiophenes(THs)asshowninTable1[6].

When faced with more severe pollution control policies, countriesworld-widehavevalidatedthatthedischargeofsulphur concentrationshouldbebelow50ppmorevenless.In orderto reducethesulphurcontenttomeetthestringentenvironmental regulationrequirements, significant attention hasbeen paid to

developing efficient processes for manufacturing cleaner fuels [7–9].Theconventionaltechniquetoremovesulphurfromfuelis knownashydrodesulphurization(HDS).HDSishighlyeffectivein eliminating acyclic and aliphatic sulphur compounds such as sulphides,disulphidesandthiols,byconvertingthemtohydrogen sulphide[10,11].Nevertheless,thistechnologyisoperatedathigh pressures(3–6MPa)and temperatures(200–450C),usescostly catalysts,andrequireshydrogen,whichmakesitlesscapableof decreasingsulphurcontent[12,13].Furthermore,theHDScatalysts have difficulties in reducing some of the recalcitrant sulphur compoundssuchasdibenzothiophene (DBT)and itsderivatives duetotheirsterichindrance[11].HDSprimarilyproceedthrougha hydrogenationroutewithrespecttothosesterichindrance.Even though the supported noble metal catalysts perform well in hydrogenation, they are easily poisoned and deactivated by sulphur.ToovercomethoseproblemsandtoacceleratetheHDS process,alloyingprocessestoreducesulphursensitivityandacidic supports thatallowdealkylationand isomerizationof thealkyl groupswereused.Thisproducesmorereactivespeciesfromthe refractorycompounds[14].DuetotheineffectivenessofHDS,the developmentofbettermethodsfordeepdesulphurizationoffuels is attracting more attention. It is crucial to introduce other approachestodesulphurization,suchasextractivedesulphuriza- tion (EDS),adsorptivedesulphurization (ADS),biodesulphuriza- tion(BDS),andoxidativedesulphurization(ODS).

EDSisawidelyusedtechnologysinceitcanbeperformedat ambientpressureand temperature.Theimportant criteriafora

*Correspondingauthorat:SchoolofChemicalandEnergyEngineering,Facultyof Engineering,UniversitiTeknologiMalaysia,81310UTMJohorBahru,Johor,Malaysia

E-mailaddress:aishahaj@utm.my(A.A.Jalil).

https://doi.org/10.1016/j.jiec.2019.02.024

1226-086X/©2019TheKoreanSocietyofIndustrialandEngineeringChemistry.PublishedbyElsevierB.V.Allrightsreserved.

ContentslistsavailableatScienceDirect

Journal of Industrial and Engineering Chemistry

j o u r n a l h o m ep a g e: w w w . e l s e v i e r . c o m / l o c at e / j i e c

goodextractantorsolventincludegoodextractivecapability,high purity, environmental gentleness, non-toxicity, and reusability [15].Manytypesofextractantssuchaswater,N,N-dimethylfor- mamide,acetonitrile,methanol,anddimethylsulfoxidehavebeen exploredinEDS[16].However,theemergingenvironmentaland safetyissues,suchaswastewateremissionandfirehazardsthat arisefromtheflammableandvolatileorganiccompounds,have becomeworldwideconcerns.Besides,EDSefficiencyina single extractionisstillnotverysatisfyingandmultipleextractionsare typicallyessentialtogetthestipulatedsulphurcontentinfueloils [17].Sincetheabove-mentionedcommonorganicsolventsdidnot achieve promising results and have their own limitations of environmentalissues andreusability,newextractionagentsare beingwidelysearched.

Adsorptivedesulphurization(ADS)hasseveral advantagesin termsof scalabilitytodesulphurizehydrocarbonfuels toa low sulphurconcentrationwithouthydrogenrequirement.Itcanbea polishingstep for theHDS processand provides an alternative methodtoreducetheproductioncostofcleanerfuelswhichcan competentlyremoverefractoryaromaticsulphurcompounds[18].

Thechoiceofadsorbent forultra-deep desulphurizationisvital becausethematerialprovidesanactivesurfaceareawithahigh volumeofproper pore size[4]. Besides,theimportantrequire- mentsforanefficientadsorbentincludeafacilesynthesismethod, mild preparation conditions, high regeneration capability, and environmentalpracticability[19].Sincethesurfaceofthemicro/

mesoporesofadsorbentscanbemodifiedusingvariousmethods,it ispossibletooffersuchporosityandsurfacecharacteristicsthat areimportantcriteriaindesulphurizationperformance.However, sometimesthemainproblemswiththecommonadsorbentsisthat theirstructure,porosity,andsurfacefunctionalityarenotexplicitly defined, and therefore it is difficult tomodifythem to meet a specificrequirement[19].Consequently,muchresearchhasbeen dedicatedtoprovidingnewadsorbentswithpromisingadsorption capabilityandexcellentselectivityandreusability,aswell asto clarifyadsorptionmechanisms[20].

Microbialdesulphurization,alsoknownasbiodesulphurization (BDS),involveasimpleinstallationprocess,lowenergyconsump- tion,mildreactionconditions,lowoperatingcosts,highlyselective removalofsulphurcompounds,andlessformationofundesired

sideproducts[22,23].InBDS,DBTiscommonlyusedasamodel sulphurcompound.ManymicroorganismshavebeenusedinBDS including Rhodococcus erythropolis H-2; Rhodococcus sp. IGTS8;

Mycobacteriumsp.G3;PseudomonasdelafieldiiR-8;Bacillussubtilis WU-S28;Microbacteriumsp.ZD-M2;andMycobacteriumpheliWU- F1,etc.Nonetheless,BDSpathwaysarenotcommerciallyviablefor petroleum fuel because a noteworthy amount of carbon is mineralized which reducesthe fuel value [24]. A considerably new approach is required to commercialize it, including by increasing specific desulphurization activity, removing sulphur at higher temperatures, phase tolerance of hydrocarbon, and segregating new strains to remove more types of sulphur compounds[25].

Among the above-mentioned desulphurization technologies, oxidativedesulphurization(ODS)isregardedasoneofthemost encouraging techniques for meeting the environmental regula- tions forsuperdeepdesulphurizationof fueloil [26].Themain benefitofODSisitsmildoperatingconditionswhichincludelow temperatureandambientpressure,aswellasitshighabilityto oxidize andconvert mostrefractorysulphur compoundsuchas DBTintheabsenceofhydrogen[27,28].Inordertoimprovethe performance of ODS, other assisted techniques are usually employedsuch asadsorptive-, extractive-,microwave-,electro- chemical-,ultrasound- andphotocatalytic-ODS[29–34].Besides, variouscatalyticmaterialsincludingorganicacids,heteropolyox- ometalates, ionic liquids, molecular sieves, and photocatalysts have been explored for ODS [35]. To date, numerous types of oxidants havealsobeenused,suchasNO2,O3,H2O2,andsolid oxidizing agents[36]. It is worthmentioningthat theoxidized products from ODS can be removed by a polar solvent like acetonitrile,water,methanol,etc.,duetotheirhigherpolaritythan sulphur[37].

Inmicrowave-assistedODS,thereactionsolutionexperiencesa possibleselectiveandrapidheatingascomparedtoaconventional heatingprocess[38].Thisisduetothenatureofmicrowaveswhich canpenetrateanddisperseenergyinthematerials,aswellastheir abilitytogenerateheatuniformlythroughoutthematerials,thus causingrapidheatingforanefficientandquickheatingprocess[39].

Profitingfromthatadvantage,apromisingperformancecouldbe attained using microwave-assisted ODS under milder reaction conditions[40].Mesdouretal.reportedmicrowave-assistedODS usingionicliquids(ILs)inametal-catalyzedoxidationthatoperated atthreedifferentmicrowaveoutputpowers,whichare150,350,and 500W[41].Theresultsdemonstratedthatincreasingthemicrowave irradiation power improved the desulphurization efficacy, and 500Wgave the highest performance(86.67%) within 90s. It is noteworthy that microwave radiation produced two types of energiesofactivation;(i)quickoxidationandthermaldecomposi- tion of catalyst with the highly polar oxidant for accelerated desulphurization performance and (ii) thermal energytransport from highly polar sulphur compounds to non-polar sulphur compoundssuchasDBTanditsderivatives,withimprovedsulphur removal. It was reportedthat the staticstate of dipoleoxidant molecules mightbe excited upon microwave irradiation, which wouldbeabletoinducethemolecularskeletonofacompoundtoa higherenergystate,weakentheCSbondandfurtheroxidizedby theoxidanttoformsulphoxidesandsulphone[40].

Electrochemicaloxidativedesulphurization(EOD)hasreceived attentionrecently,duetoitsabilitytoreducethesulphurcontent atmildtemperatureandpressurewithouttheadditionofoxidants [42].Also,itwaspossibletocontrolthedesulphurizationproducts and reactionefficiencybyusingdifferentpotentialand current.

Tangetal.proposedanEODmethodforsulphurcontentreduction in kerosene,whichwas performedinNaClsolution[43].Itwas recognizedthatCloftheNaClelectrolytewasoxidizedtoformCl2

and ClO that are able in oxidizingthe sulphur compoundsto Table1

Typeofsulphurcompoundsinfuel.

Sulphurcompound Chemicalstructure

Mercaptans R–S–H

Sulphides R¹–S–R²

Disulphides R¹–S–S–R²

Thiophene

Benzothiophene

Dibenzothiophene

sulphoxideand sulphone.Inthat study,N-methyl-2-pyrrolidone (NMP)wasusedtoextracttheoxidizedproductsfromkerosene.

Theresults demonstratedthat 1-heptylmercaptan as themain sulphurcompoundinkerosene,wasefficientlyoxidized,whichcan be separated by extraction. After EOD, the sulphur content in kerosenewasreducedto13.2

m

^gg1with92.7%desulphurization efficiency.Though,itisworthmentioningthatthistechnologystill requirefurtherresearchtopushittowardcommercialization.

Similarly, ultrasound assisted desulphurization (UAOD) also couldbeperformedundermildconditionswithoutusingexplosive hydrogen[44].Ithasbeenreportedthattheoxidationofsulphur compoundsoccursinthebulkofasolventorattheinterface,which obviouslyneedswelldispersionofbothsolventandfuelphases.

Forthatpurpose,anultrasonicpulseisusedtocreateverysmall dropletswithhighdispersionofthosetwophases.Besides,water inaqueoussolutionscouldbeeasilydecomposeunderultrasound toform hydroxyl radicals and hydrogen peroxide, which could partlyoxidizesulphurcompoundsintosulphones. Furthermore, sonication also can directly decompose the semi-volatile and volatile thiophenes and thioethers in aqueous solutions [45].

Benefitingfromthat,ahigherreactivityofthiophenecompounds wasachievedduringUAOD (98.4%),which convertedtheminto highlypolarsulphoxidesandsulphonesthatweresimplyremoved byadsorptionorextraction[44].

Due to several limitations of those above-mentioned ODS methods in terms of their application and commercialization, photocatalyticODShasbecomethemainareaofattentionamong researchersnowadaysforfurtherexplorationandinvestigation.

Photocatalyticoxidativedesulphurization(PODS)

Generally, photocatalysis is a process that requires light to activateacatalystforspeedingupchemicalreactionsinorderto solvenumerousairandwaterpollutionproblems[46].Asoneof theadvanced oxidationprocesses (AOPs),it hasreceived much scientificattentionsinceitwasfirstdiscoveredbyFujishimaand Honda in 1972. This is due to its outstanding performance in degradingawiderangeoftoxiccompoundsunderlightirradiation, aswellasitshugepotentialinbothindustrialpretreatmentand full-scale treatment [47–49]. The other advantages of photo- catalysisover other treatment methods are its easy-operation, low-cost, sustainable technology, environmental friendliness, capabilityofperformingatroomtemperature,anditsability to transformtoxiccontaminantsintoharmlessproductswithhigh selectivity[50–53].

In recent times, semiconductor photocatalysisutilizing solar light have attracted worldwide interest for solving numerous environmental issues, mainly due to their high-performance, energy-efficiency,andeconomicproperties[54].Duetoitshigh percentage of solar light content (50%), visible light is easily acquiredacrosstheearth.Thus, significantrecentattentionhas beengiventovisible-lightorientedphotocatalysis[55].Specifical- ly,thenecessaryrequirementsforeffectivephotocatalysisarewide lightabsorptionrangeandstrongredoxcapability[56].Neverthe- less, the main problems with single-component photocatalysts includealimitedlightresponserange,inefficientchargesepara- tion, fast electron–hole recombination, and weak redox ability [56,57].Ithasbeenstatedthatanarrowbandgapisrequiredto obtainanextended light absorptionrange;while for improved redoxcapability,amorepositivevalenceband(VB)andnegative conductionband(CB)isneeded[56].Todate,controllingbothof thesecriteriastillremainsasahugechallengeforresearchers.

AsoneoftheODSmethods,photocatalyticoxidativedesulphu- rization(PODS)isconsideredtobeapromisingtechnique.Dueto itshighcatalyticactivity,safety,lowenergyconsumption,andlow cost,itisahighlyeffectiveandsimpleprocess[58–60].Moreover,

PODS isa greenalternativethatis abletotransform hazardous sulphurcompoundsintoenvironmentallyfriendlysubstances[61].

This is a differentapproach when compared to ADSor toEDS methods that only adsorb or extract the sulphur compounds, respectively.Adistinctivecriterionincludingphoton-activationof achemicalreactioninsteadoftemperatureactivation,aswellas theuseofplentifulandeffectivelypermanentsolarenergy,also leadstothegreatpotentialofPODS[62].Nowadays,manystudies havebeencarriedouttodevelopactiveandselectivecatalystsfor PODSofliquidfuel.Ithasbeenpreviouslyreportedthatvarious criteriaincludingcatalysttype,morphologicalproperties,surface area, bandgapenergy,surface defects,and process parameters suchascatalystdosage,initialconcentration,aswellasoxidant andextractant usageare amongthevitalaspectsforimproving overallPODSperformance.Remarkably,manypreviousoutstand- ingreviewsandviewpointshavediscussedtheODSprocessalone [63,64]orincludeditalong withotherdesulphurizationtechni- ques[65,6].However,tothebestof ourknowledge,reviewsof PODSarestillscanty,inspiteofthefactthatintensiveresearch effortshavebeenmadetorevealthevitalaspectsrelatedtoPODS catalystsrecently.Thus,thisarticleishopedtoprovideinsightinto thestrategyandprogressofefficientPODScatalyticperformance.

Photocatalysts

Over thelastfew decades,variousphotocatalysts havebeen reported, andsemiconductor-based catalystssuchas zincoxide (ZnO),titaniumoxide(TiO2),tungstenoxide(WO3),etc.havebeen developedforPODSduetotheiroutstandingperformanceinPODS [66].InPODS,theadditionofaphotocatalystcouldpromotethe oxidationofsulphur-containingcompoundsbyenhancingtherate ofactivespeciesformationandbyimprovingchargetransfer.From previousstudies,itwasperceivedthatthesulphurremovalwas instantaneousincreasedbyincreasingtheamountofphotocatalyst [67,68].Nevertheless,furtheradditionabovetheoptimumvalue contributedtoadecreaseinsulphurremoval.Thisismostprobably due to thefact that a superfluous amountof catalyst tendsto hinderthelightsourceanddecreasesphotoelectronproductivity, thusinfluencingthephotocatalyticoxidationefficiency[69].

Itis worthmentioningthatotherthantheabove-mentioned problem,themaindifficultyfacedbyPODSisinfindingasuitable catalystthatishinderedfromthefollowingproblems;atendency to agglomerate in the reaction system, a fast electron–hole recombination rate, as well as a wide band gap and narrow spectrum utilization [70,71]. These shortcomings decrease the activesitesurface,reducephotocatalyticactivity,andthishasled toalimited rangeofapplications[72,73].In themeantime, the inertnessofsulphur compoundsisstrongly dependentontheir electron density and aromaticity, which makes photooxidation quite challenging[74].It hasbeenpreviouslyreportedthat the differentselectivity betweenthesulphur compoundswas often ascribed totwo sources, which are theelectron density of the sulphur atom in the compounds and the steric-hindrance of substitutedmethyl groups. Bothof theseare electron donating groups(EDG),whichmightcompetetoformabond[75].

Decorationofphotocatalysts

ResearchershavesoughttoimprovePODSinnumerousways andtherehasbeenconsiderableprogressinthemodificationofthe existingcatalysts.Generally,thereareseveraltechniquesthathave been previously reported to improve photocatalytic efficiency:

(i)synthesisofhighsurfaceareamaterials,(ii)theuseofasupport material,(iii) couplingwith organicand/or inorganicmaterials, (iv)thecombinationoftwoormoredifferentsemiconductors,and

(v) formation of a defect structure to improve charge-carrier separation[76,68,77].

Theuseofsupportmaterials

Asagreatphotocatalyst,TiO2isextensivelyusedtooxidizeDBT and itsderivativesinfuel andconvert them intosulphoneand sulphoxide.ThewideutilizationofTiO2inPODSisstronglydueto itspromising photoactivity, chemical stability,non-toxicity and lowcost[70,77].Nonetheless,itsphotoactivityisrestrictedbyits hightendencytoagglomerateinthereactionsystembecauseof thenaturalsmallsizeofTiO2particles.Otherthanreducingthe surfaceactivesitesandinhibitingtheactivitytowardsPODS,the separationprocessofnano-sizecatalystsbyfiltrationalsorequires highcosts [78]. In orderto overcome those problems, support materialshavebeenusedtodisperse andimmobilizeTiO2.The utilizationof a zeolite framework asa supportis an attractive approach.Thisisduetoitsphotochemicalandchemicalstability, high thermal resistance, high surface area, high adsorptive capacity, shape selectivity, and quantum confinement effect [79,80–82].Moreover,azeoliteframeworkcanactasadispersing agentwhichpreventstheagglomerationofcatalystandincreases the amount of surface active sites [83]. Therefore, research focusing onthesynthesis of TiO2-zeolitecomposites(TS-1) for variousphotocatalyticreactionshasgainedmuchinterestrecently [84,85].InthepreviousPODSstudy,theperformanceofTS-1was higher thanthat of TiO2,mostly due tothegood dispersionof isolatedtetrahedralTiO2speciesintheframework[86].Thegood dispersionof TiO2 particles provides a large surface area, thus offershighavailabilityofactivesitesforefficientlightabsorption aswellasforsulphuradsorption.Thisleadstotheformationof manyactiveradicalspeciesuponlightirradiationwhichareableto reactwithDBTmoleculesforenhancedPODSactivity[87].

EarlierstudiesrevealedthatthemesoporousM41materialsare widelyutilizedascatalystsupportsduetotheirpromisingsurface andtexturalcharacteristics[88–90].Asoneofthem,mesoporous silica(MCM-41)demonstratesa hexagonalarrayofconsistently ordered channels with a significant number of SiOH groups which makes it a suitable candidate for distributing TiO₂ by controllingtheparticlesizeandeffectivelyretainingTiO2particles ontheMCM-41framework [78]. For thatreason, MCM-41was usedtosupportTiO2inaself-dopedcarbonTiO2@MCM-41(CTM- 41)[91],whichwasthenusedinPODSofDBT.Itwasreportedthat theuniformorderedchannelsofMCM-41caninhibitagglomera- tionofTiO2bycontrollingofitsparticlesize(Fig.1).Besides,the largesurfaceareaandappropriateacidityimprovedtheadsorption ofsulphurcompounds,thereforeenhancedphotocatalyticperfor- mance[87].Inaddition,X-rayphotoelectronspectroscopy(XPS) andultraviolet(UV)analysesdiscoveredthatanothercontribution tothehighphotocatalyticactivitywastheextendedabsorptionof

light, which is duetothepresence ofinterstitial carbonin the structure fromtheresidueof Tiprecursor. Itwas observedthat 95.6%photocatalyticdegradationofDBTcouldachievedusingthat catalyst.Duringthereaction,thenarrowedbandgapenablethe excitationofelectronuponvisiblelightirradiation,thusproduce thesuperoxideanionsafterthoseelectronreactwithO2,which thenformtheOH.Moreover,thehighabsorptionofphotondueto theimprovedsurfaceareawouldalsoencouragetheformationof thoseactivespecies.

Besides, MCM-41was also applicabletothe dispersionof a surfaceplasmonresonance(SPR)catalysttopreventitsagglomer- ation probability [72]. Recently, the SPR property displayed by somenoblemetals,includingAgandAu,hasbeenmanipulatedto increase visible-light harvesting [92]. Some catalysts with promising photocatalytic activities have been developed based onthatcriterionincludingAg/TiO2[93,71],Au/TiO2 [94],Au/SiC [95],AgBr[96],etc.Amongthese,AgBr(2.6eV)wasestablishedto beaneffectivevisible-lightphotocatalystforthedegradationof organic compounds, due to its outstanding photosensitive capabilities[96,97].Besides,it isworthmentioningthatfurther combinationofAgwiththeabove-mentionedAgBrtoformAg– AgBrcouldenhancetheperformance,duetothesymbioticeffect between the SPR of metallic Ag⁰ with the photosensitive characteristicofAgBr[98].Inordertopreventtheagglomeration of Ag–AgBr and further improve its photoactivity in terms of improvedsulphurcompoundandphotonadsorption,theuseofa highsurfaceareasupportwasfoundtobecrucial.Consideringthat, 10–60%Ag–AgBrwasdopedonAl-MCM-41andthenusedinPODS of dibenzothiophene [72]. The scanning electron microscopy (SEM)imagesshowedthattheAg–AgBrnanocompositeobtained wasdisperseduniformlyonthemesoporouschannelofMCM-41.It alsowasfoundthatthedesulphurizationrateincreasedfrom78%

to98%byusing10–40%Ag–AgBr/Al-MCM-41. However,therate wasdecreasedforboth50–60%Ag–AgBr/Al-MCM-41.Theseresults revealedthat a smallamountof Ag–AgBrisenough toprevent recombination of electron–hole pairs for improved charge separation. Conversely, the higheramount ofAg–AgBrledtoa formation of huge amount trapping sites which subsequently assistedthephoto-productioncarriers’recombination.Uponlight irradiation,AgBrandAg⁰areexcitedtogeneratephotogenerated electron–holepairs.Besides,theAg⁰nanoparticlesalsoplayeda role as electron trapper to avoid the electron hole pairs’ recombinationforenhancedperformance.

Inordertoimmobilizethemetaloxideparticles,afoldedsheet mesoporous silica (FSM-16) support was also used to provide substantialenhancementofperformance.Itwasreportedthatthe importantcriteria ofFSM-16includesanabundantsurfacearea (>1000m²g¹), bigger pore sizes, a homogeneous mesoporous sizeofone-dimensionalpores,andhighthermalstability.Thishas

Fig.1.SchematicreactionbetweenMCM-41andtitaniumsource.AdaptedfromRef.[91].

increaseditsapplication inthefieldof catalysis.Based onthat specialty,FSM-16wasusedtodispersezincoxide(ZnO)forPODSof DBT [49]. ZnO is a cost-effective, non-toxic, highly stable, and environmentallyfriendlysemiconductorforphotodegradationof organiccontaminants[99].Nevertheless,thewideZnObandgap (3.37eV) inhibitsthegenerationof chargecarriersupon visible light illumination [100,101]. The photoluminescence (PL) and diffuse reflectance spectroscopy (DRS) analyses show that the incorporationofZnOontheFSM-16reducedtherecombinationof electron–holeandimproved theabsorptionofvisible light.The performanceofthephotocatalystsweremeasuredundervisible and UVlight irradiation.It was foundthat FSM-16shows very limitedactivityunder eithertypeof light,this wasfollowedby pureZnO,whilethehighestperformancewasobtainedusingZnO/

FSM-16.ThehigherefficacyshownbyZnO/FSM-16thanpureZnO wasprobablyduetothegooddispersionofZnOspeciesonthehigh surface area of FSM-16. This enhanced light harvesting and improvedtheinteractionofthegeneratedradicalswiththeDBT molecules.

Thephotocatalyticprocessovercarbonnanomaterial(CNMs)- modified catalysts is an effective solution for remediation of variousenvironmentalpollutants. Notably,asone oftheCNMs, carbonmultiwallnanotubes(MWNTs)aresomeofthesupports thataremostoftenusedtoenhancethephotocatalyticperfomance ofagglomeratedmetaloxides[102].MWNTshavebeenextensively studiedduetotheirbeneficialpropertiessuchasnanosize,large surfacearea,highadsorptioncapability,goodelectrical conduc- tivity,highchemical andthermal stability,excellentmechanical properties,and well-defined hollowinteriors. Thisimplies that they have huge potential application as supports for nano- composites [103–105]. In addition, the large electricity-storage capacityofMWNTsenablesthemtotrapexcitedelectrons,thus inhibitingelectron–holerecombination[106].Inanotherrelated study,Ag–TiO2/MWCNTswasusedforPODSanditwasfoundthat bothTiO2and Agnanoparticlesweredispersedhomogeneously overthe MWCNTs[67]. ThePODS of thiopheneachieved100%

using1.4gL¹ofcatalystina600mgL¹solutionwithin30min.

Duringthereaction,Agdopingpreventedtheelectron–holepair’s recombination, while MWCNTs increased the light absorption propertiesof TiO2.Aftertheexcitationofelectronoccurred,the excited electron was trapped by the Ag, thus enhanced the electron–holeseparation.

As one of the natural clays, montmorillonite (MMT) is extensivelyused for photocatalysis applications in the form of semiconductor-claynanocomposites[107].MMTis anabundant and low cost clay mineral with 2D sheet-like morphology containing hydrated magnesium aluminum silicate and it pos- sesses largesurface area[108,109]. These uniquefeatures have made it a promising support material in order to avoid the agglomeration problem and to construct high performance catalyticcomposites.Inordertoreducetheaggregationprobability

ofBi2W_1xMoxO6/Bi2MoO6 heterostructure,MMTwas usedasa supportinaone-pothydrothermalmethod[110].Thiscatalystwas thenefficientlyusedinphotocatalyticdesulphurizationofDBT.In the other similar study, attapulgite (ATP), a 1-D fibrous nano- structureofnaturalmagnesiumaluminumsilicate,wasusedasa supporttodispersetheBiP_1xVxO4/BiVO4forPODSofDBT[111].

Thissupportwasselectedduetoitshighsurfacearea,excellent adsorptivecapability,anditsparticularmicroporousstructure.It wasobservedthatthedesulphurizationperformanceundersolar lightattained90%.Thiswasduetotheroleofthelargesurfacearea oftheATPsupportforimprovedadsorptionabilityandbettersolar light harvesting by the staggered heterostructure BiP_1xV_xO₄/ BiVO4thatalsoenhancedthechargemigrationandthusenriched thedesulphurizationperformance.Table2depictsasummaryof thePODSofsulphur-compoundsusingdifferenttypesofsupport materials.

Based ontheabove-mentioned observations and findings, it couldbeconcludedthatasuitableselectionofhighsurfacearea support materials might play crucial roles in enhancing the performance of PODS of sulphur-containingcompounds. These supportmaterialswerefoundnotonlytofunctionasdispersants forthemetaloxideparticles,butalsotoimprovetheadsorption capabilityofthetargetedsulphur-compoundsandphotons,thus enhancingtheperformance. Fig.2 illustrates theadvantagesof using a support material to prevent the agglomeration of a photocatalyst.

Dopingwithmetalelements

Bismuthvanadate(BiVO4)asapromisingvisiblelightphoto- catalyst,couldofferabigporesizeandhighsurfacearea,whichare advantageousfor improving thecontact of catalyst active sites withthesubstrate,thusdemonstratingoutstandingperformance [70].Nevertheless,theuseofpureBiVO4isrestrictedbyitspoor adsorptioncapabilityandphotocatalyticeffectivenessisinhibited by inefficient charge carrier separation [112]. Considering that, metalormetaloxideshavebeenloadedontoBiVO4 inorderto enhancetheseparationofthechargecarriers.Italsowasreported thatthemetalormetaloxidedopingwasnotonlyapplicablefor chargecarrierseparation,butitwasalsoabletoextendtheband absorptiontothevisibleregionbynarrowingthebandgap[87].In aPODSstudy,Ag–BiVO4washydrothermallysynthesizedandthe as-preparedcatalystdemonstratedanexcellentabilitytoharvest visiblelight,aswellasbetterperformancethanpureBiVO4[11].

This was due to the distribution of Ag particles on the BiVO4

surface,whichinhibitedthephotogeneratedelectron–holepairs’ recombinationandaddedmoreactivesitesforthiophenephoto- degradation.Additionally,thepHvalueduringsynthesiswasalso foundtobecrucialforthatpurposeandpH7wasclaimedtobethe bestcondition,duetotheformationofamixturehighsurfacearea ellipsoidal structures and “bird’s nest” morphologies. This properties contribute to the high adsorption capability of

Table2

SummaryoftheresultsfortheuseofsupportmaterialsinPODS.

Photocatalyst Support Synthesismethod Lightsource Experimentalconditions Photocatalytic

performance

Ref.

C/TiO2 MCM-41 Reflux 300Wtungstenlamps(visible

light)

Catalyst=1.5gL¹[DBT]=300mgL¹ 95.6%in300min [91]

Ag–AgBr Al-MCM-

41

Chemicaldeposition 125WhighpressureHg(visible light)

Catalyst=20.0gL¹ [DBT]=500mgL¹

98%in360min [72]

ZnO FSM-16 Ionexchangeprocess Hglamp(60W,UV) Catalyst=0.3gL¹[DBT]=200mgL¹ 95%in550min [49]

Ag–TiO2 MWCNT Modifiedacidcatalyzed sol–gel

500Wxenonlamp(visiblelight) Catalyst=1.4gL¹ [Thiophene]=600mgL¹

100%in30min [67]

Bi2W1xMoxO6/ Bi2MoO6

MMT One-pothydrothermal 300Wxenonlamp(visiblelight) Catalyst=nodata[DBT]=200mgL¹ 95%in180min [110]

BiP_1xVxO4/BiVO4 ATP Microwave-hydrothermal 300Wxenonlamp(visiblelight) Catalyst=nodata[DBT]=200mgL¹ 90%in180min [111]

sulphur-compounds, as well as more absorption of photon, thereforeprovidedmoreelectron–holepairsandadditionalactive sitesforenhancedperformance.

Recently,ZrO₂hasbeenacknowledged asa potential photo- catalyst among various semiconductors for use in advanced oxidation processes (AOPs), due to its chemical stability and suitable redox potential [113]. Nonetheless, its wide band gap (5.0eV) and fastrecombination of charge carriershamperits photocatalyticeffectiveness.Thisrequiresvariousmodificationsto extenditsapplicationrange[114,115].Itwasreportedthatthemost common way to suppress electron–hole recombination and increasethe proficiencyofa photocatalyst isby dopingit with transitionmetalcationssuchasPd,Ag,Pt,Ni,andCu[115].By considering that factor, Pd/ZrO2 was doped onto chitosan by a modifiedsol–gelmethod.ThiswasdenotedasPd/ZrO2–chitosan and it was used in PODS [68]. As a versatile and abundant polysaccharide derived fromthepartial deacetylation ofchitin, chitosanisanidealcandidatefordispersing Pd/ZrO2 [116].Asa result,thesynthesizedPd/ZrO2–chitosancatalystwaspresentin nanosized dimensions; it was comparatively uniform and dis- persedhomogeneously [68].In that study, it was claimed that chitosanactsasa promoterinthecrystallizationprocesswhich inhibitsagglomerationdue tothecalcinationprocess,while Pd increase the sensitivity of ZrO2 towards visible region. This photocatalystcanbeusedefficientlyundervisiblelightirradiation with100% thiophene degradationusing 0.9gL¹ of the photo- catalystfor 500mgL¹ thiophenesolution.Thiswasdue tothe improved adsorptioncapability of thiopheneand absorptionof photon,duetotheenhanceddispersionofPd/ZrO2bychitosan,as wellastheroleofPdinsuitingthebandgapofthecompositeto meetthevisiblelightenergyrequirement.

Asawell-knownphotocatalyst,TiO2hasalwaysbeenopenfor modification opportunities in order to gain better activity in

variousphotocatalyticapplications.Itwasreportedthatoneofthe importantcharacteristicstomodifyforimprovedphotocatalytic activity ofTiO2 isitsphasestructure [117].AmongTiO2phases, anataseis the mostwidelystudiedfor photocatalyticreactions [118,119]. In order to maintain the highly photoactive anatase phase TiO2 with a tunable surface phase, Lin et al. calcined commerciallyavailableTiO₂withdepositedsodiumsulfateNa₂SO₄ asamodifiertoformsulfatedTiO2(SO42–TiO2)[74].Tothebestof our knowledge,this is thefirst study that reported theuse of sulfatedTiO₂forthePODSofthiophene.Ontheotherhand,itwas foundthattheadditionofasmallamountRuO2asanoblemetal oxideontoTiO2couldgreatlyenhanceitsphotocatalyticactivity.In thatstudy,RuO2actsasanoxidationco-catalyst,whiletheSO42– TiO2 could capture superoxide species and activate thiophene molecules, due to its Lewis acid nature [120]. The synergistic relationshipofbothmaterialsisbeneficialfortheefficientPODSof thiophene.InthemechanismgiveninFig.3,thephotogenerated electronwasreactedwithO2onthesurfaceofthephotocatalyst afterbeingirradiatedbylighttoformsuperoxidespecies(O2

and O₂²). As a typical Lewis acid, SO₄²–TiO₂ accepts the photo- generated electrontoproduce ananion radical, which can add more superoxide species (O2

and O22). Meanwhile, as an oxidationco-catalyst,RuO₂acceptstheholefromVBofSO₄²–TiO₂ thatwouldactasanoxidant.Asummaryofthedopingofmetal elementsonthePODScatalystsisshowninTable3.

Couplingwithothersemiconductors

As has beenmentioned in the previous section,the photo- activity ofa singlecomponentcatalystis alwayslimitedbythe rapidelectron-holesrecombination,whichleadstolowquantum efficiencyandpoorphotocatalyticperformance[121].Itiswell- known that the development of a heterojunction structure between a photocatalyst and a suitable band potential Fig.2.Pre-andpost-additionphenomenonofsupportmaterialtothePODScatalyst.

semiconductor may provide an appropriate way to solve the problemsaccordingtoanarrowphoto-absorptionrangeandafast photogeneratedchargecarrierrecombination[122].Inahetero- junction structure, the energy gap between the two different semiconductors prevents the recombination of photogenerated electron–hole pairs by permitting the transfer of those charge carriersfromtheenergylevelofonetypeofsemiconductortothe energylevelofanother[123].Thisconsequentlycontributetoan efficient and long-term charge separation. For that reason, nowadaysmoreresearchesarefocusingontheconstructionofa heterojunctionforabettercatalyticperformance.

Then-typesemiconductorceria(CeO2)isoneofthepromising materialsamongtheemergingcatalysts,duetoitshighcatalytic activity,lowcost,environmentalfriendliness,richoxygendefects, and superior redox ability [124,125]. However, the exclusive photocatalyticperformanceofCeO2 islimitedbyitswideband gapandlowelectron–holeseparationefficacy[126].Numerous approachestoenhanceitsphotocatalyticactivityhavebeenmade includingthereducingofparticlesize,adjustingthecrystalfacets and morphologies, doping with metallic or non-metallic ele- ments,aswellasfabricatingheterojunctions[127,124].InPODS studies,CeO2 wasfabricatedwithvarious materialsinorder to improveitsphotoactivity[127–129].Lietal.incorporatedCeO₂ withATPbya microwavemethod, followedbytheadditionof modified graphiticcarbonnitride (g-C3N4)bya simpleelectro- static-induced self-assembly technique (Fig. 4) [127]. Unlike a metalcompoundphotocatalyst,g-C3N4canbeeasilyproducedby heatingnitrogen-containingprecursorssuchasurea,melamine, thiourea, or cyanamide [130,131]. Currently, g-C3N4 has been comprehensively used in various photocatalytic applications includingdegradationoforganicpollutants,hydrogengeneration, CO2reduction,aswellasfororganicsynthesis[132].Asa non- metalphotocatalyst,g-C3N4hasbeenestablishedasanon-toxic

andlowcostphotocatalyst withexcellentoptical,thermal,and electricalcharacteristicsandhighchemicalstabilitythatisuseful in various applications [133–136]. However, it only possesses moderate activity because of its low surface area, excitonic effects,inadequatevisiblelightabsorption,rapidchargerecom- bination,aswellasinefficientchargetransferthathaslimitedits practical application. Thus there is a need for heterojunction formationwithothersemiconductors[137–139].Previousstudies haverevealedthatg-C₃N₄isagoodcarrierwhendopedwitha series of materials for improved catalytic activity [140–142].

Another importantcriterionof g-C3N4 is thenarrow band-gap (2.70eV)whichenablesittoexcite,thusformphotogenerated electronholepairsuponvisiblelightirradiation.Thismakesita suitablecandidateasaphotosensitizerinnumerousapplications [143,144].Benefitingfromthenarrowerbandgap(2.70eV)and morenegativeCBofg-C3N4thanCeO2enablesitsutilizationof visiblelightandapossiblemigrationofphotogeneratedelectrons fromtheCBofg-C3N4totheCBofCeO2.Meanwhile,asmentioned before in a previous section, ATP served as a framework to immobilize both materials and simultaneously improved the adsorptioncapability.Thesynergisticeffectbetweentheabove- mentioned materials has led to outstanding visible light performanceandexcellentstability.

Apartfromthat,itisaninterestingfindingthattheVBandCBof g-C₃N₄ (VB:1.57eV; CB: –1.12eV) and TiO₂ (VB: +2.91eV; CB:

–0.29eV) meet the heterojunction formation criterion for en- hancedphotocatalyticactivity[145,146].Forthatreason,inoneof thePODS,g-C3N4was incorporatedwithTiO2 [147].Theresults demonstratedthat TiO2/g-C3N4presentsa considerableinterde- pendenteffectbetweenthosetwomonomers.Duringthereaction, the photogenerated electrons from CB of g-C3N4 were quickly transportedtotheCBofTiO2,whiletheholefromVBofTiO2was transferredtoVBofg-C3N4,thispreventedtherecombinationof electron–holepairs.OtherthancouplingwithCeO2andTiO2,there isalsoareportontheuseofg-C3N4incorporatedwithBiVO4for PODS [70]. For a further enhanced performance, a g-C3N4

nanosheetwasdistributedinmesoporoussilicachannels,which werethenloadedwithBiVO₄byahydrothermalmethodtoforma BiVO4/C3N4@SiO2heterojunction.Thephotocatalyticperformance ofBiVO4/C3N4@SiO2wascomparedwithC3N4@SiO2,BiVO4@SiO2,

andBiVO4/C3N4.Itwas foundthattheDBTconversion achieved 99% using BiVO4/C3N4@SiO2, this was 2.4-times higher than C3N4@SiO2andBiVO4@SiO2,signifyingtheinterdependenteffect betweenBiVO4andC3N4.Thedifferentpositionoftheenergyband between C3N4 and BiVO4 has led to the formation of a heterojunction,whichincreasedtheadsorptionintensityofvisible light and suppressedthecharge carrier’srecombination.In the meantime,BiVO₄/C₃N₄@SiO₂demonstratedatripledDBTconver- sionascomparedtoBiVO4/C3N4,whichsuggestedthatSiO2also played a crucial role for an effective performance. Even the cooperationbetweenC₃N₄andBiVO₄alsoexistedintheBiVO₄/ C3N4.However,itsphotocatalyticperformancewaslimitedbythe larger particle size of BiVO4, thus requiring mesoporous silica channelstocontrolitsparticlesize.

Table3

SummaryoftheresultsforthemetaldopinginPODS.

Photocatalyst Dopant Synthesismethod Lightsource Experimentalconditions Photocatalytic

performance

Ref.

BiVO4 Ag Hydrothermal

method

400Wmetalhalidelamp(visiblelight) Catalyst=1.5gL¹[Thiophene]=500mgL¹ 95%in210min [11]

ZrO2–chitosan Pd Modifiedsol–gel 500Wxenonlamp(visiblelight) Catalyst=0.9gL¹ [Thiophene]=600mgL¹

100%in60min [68]

SO42–TiO2 RuO2 Incipientwetness method

300Whigh-pressuremercurylamp (visiblelight)

Catalyst=1.0gL¹[DBT]=600mgL¹ 88%in180min [34]

Fig.3.Schematicdescriptionofthemechanismforthephotocatalyticoxidationof thiopheneonRuO2/SO42–TiO2photocatalyst[74].

Meanwhile,Yunetal.reportedthepreparationofafour-angle star-likeCo–Almixedmetaloxide(CoAl–MMO)loadedBiVO4bya hydrothermalmethodforPODSofthiophene[112].Itwasfound thatCoAl-MMOdispersedasamorphousparticlesonBiVO4surface affected it in three unique ways; it improved visible light absorption, it formed a p-n heterojunction for charge carrier separationatitsinterfaceandBiVO4particles,anditincreasedthe negativityofflatbandpotential.Afterirradiatedbyvisiblelight, electronsonbothCoAl2O4andBiVO4wereexcited,leavingholeson bothVB.DuetothesuitablepositionofCBandVB,theelectrons migrated from CB of CoAl2O4 to BiVO4, while holes were transferred from VB of BiVO4 to CoAl2O4, thus improving the electron–hole separation and preventing the probability of recombination. Consequently, the life time of photogenerated carrierswasincreasedsothattheycouldtakemoreofapartin PODSforamoreenhancedperformancethanpureBiVO4.

Besidesitsroleasasupportmaterial,ATPalsohasbeenusedto form a heterojunction with carbon quantum dots (CQDs) for photocatalyticdesulphurizationofDBT [148].CQDs,as anewly discovered carbon material, are predicted to enhance photo- catalytic activity byconstructinga heterostructuredue totheir high absorptivity and fast electron migration properties [149].

Nevertheless,thephotoactivityofpureCQDsistypicallypoorer thanitscounterpartnanocomposites.Thisisduetothepresenceof manyemissive traps onthe surface.ATP with specific internal channelsandexcellentadsorptivecapability,aswellasabundant activesites,hasbecomeacapablesupportforothersubstances.In ordertofurtherimproveitsperformance,ATPwasmodifiedwith diff;erentconcentrationsofacidtoincreaseitssurfacehydroxyl content.ThenitwasdopedwithCQDsbyafacileimpregnation method.WhenmodifiedATP is coupledwithCQDs, itssurface hydroxyl group would hydroxylate the hydroxyl and carboxyl groups on CQDs surface, thus the CQDs nanoparticles can be attachedtotheATP(Fig.5).Theheterojunctionformationbetween bothmaterialsisnotonlybeneficialintermsofefficientcharge carrierseparation,butitalsoincreasestheutilizationeffectiveness

ofsolarlightandthuswidensthephoto-absorptionrangedueto theup-conversionpropertiesofCQDs[150].

Perovskiteshaveappearedasasignificantnewclassofmixed- oxide materials because of their excellent thermal stability, electronic structure, ionic conductivity, electron mobility, and redoxbehavior.ThegeneralformulaofperovskitesisABO3,where Aisatypicallanthanide,alkaline,oralkaline-earthcationandBis any of the transition metal cations [151]. Recently, a layered perovskiteoxideLa2NiO4,consistingofalternatelayersofLaNiO3

and LaO rock salt, has attracted considerable attention as a promisingphotocatalystbecauseofitsuniquemixedoxideionic electronic conductivity and oxygen storage capacity [152].

Moreover,acoherentsubstitutionofLa(III)withCe(IV)would introduce oxygen vacancies into the structure for facilitated oxygen transferandincreased oxygenmobility,thus improving the photocatalytic behavior [153]. Attributed to their unique physicochemical, electronic and optical properties, metalloph- thalocyanines (MPcs), macrocyclic molecules with a planar conjugated array of 18-

p

^{electrons, have been} expansively explored in numerous fields [154,155]. In photocatalysis, their photochemical abilityin electrontransferpathway toproduce radical andradical anionspeciessuchas O2

and HOand/or activate triplet oxygen into singlet oxygen (¹O₂) has aroused interest[156].Inordertoimproveitsphotostability,MPcswere loaded onto the surface of La0.8Ce0.2NiO3 (LCNO), which were then donatedasML/LCNO andused inPODSofDBT[157].The combinationofbothmaterialswasabletoreducethedegreeof electron–holerecombination,whichwasfoundtobebeneficialin enhancing thephotocatalyticperformance.Theperformance of couplingdifferentsemiconductor photocatalystsis summarized inTable4.

OtherimportantcriteriainPODS

It is worth mentioning that other than the aforementioned highlightedimportantcharacteristicsforenhancedperformanceof PODScatalysts,thereareothercrucialaspectsthatrequireintense attention.Thesuitableself-assemblysystemscontainingoxidant andextractant/adsorbentaswellasoptimizingcatalystdosageand initialconcentrationofsulphurcompoundsaresometimeessen- tialforincreasingtheefficiencyofPODS,togetherwiththespecific roleofthecatalystitself.

Oxidant

The oxidantactsastheoxidizingagentbygiving itsoxygen atom tothesulphurcompoundsintheODS process.A suitable oxidantmustbeinexpensiveandcapableofproducinglesstoxic by-productssoasnottocauseanyenvironmentalissues[58].In PODS, two common oxidantshave been usedso far;these are molecularoxygen(O2)andhydrogenperoxide(H2O2)[156].

Fig.4.FormationmechanismofCeO2/ATP/g-C3N4nanocomposite[127].

Fig.5.FormationmechanismofCQDs/ATPheterostructure[148].

ArtificialairwasusuallyusedtoprovideO₂forPODS.It was previouslyreportedthatsulphurremovalusingAg–BiVO4 could onlyachieve32%intheabsenceofO2androseto95%when30mL min¹ofairwasaddedtothereaction[11].ItwasbelievedthatO₂ adsorptiononthesurfaceofthephotocatalystwaspromptedby the photonwhich then formed the superoxide anion O² and preventedthe possibilityof electron–hole recombination[157].

EvenaninterdependenteffectbetweenthephotocatalystandO2

couldacceleratethephotocatalyticreaction.Itwas obviousthat the desulphurization rate showed a decreasing trend in the existenceofsuperfluousair flow.It was suggested thatonce it reactedwiththephotocatalyst,excessO2mighttrapaholeorOH orproduceperoxo-compoundswhichweredisadvantageoustothe PODSprocess[158].Additionally,continuousairflowwouldcause thevolatilizationofoilandthusleadtoreduceddesulphurization effectiveness[30].Besides,itcouldalsoreducetheadsorptionsites for thesulphur compound onthe photocatalyst surface, which wouldresultinlowactivityforPODS.

Generally,hydrogenperoxide(H2O2)isthemostusedoxidantin PODS because of its high oxidizing capability, environmental friendliness,anditslowformationoftoxicby-productsaswellas adequate safetyin storageand its low cost[30,64]. A previous studyshowedthatthephotocatalyticoxidationofDBTincreased byincreasingtheH2O2concentrationtoanoptimumvalue[159].

Thisisduetothegenerationofahydroxylfreeradical(OH)witha strongoxidizingpropertythatwasacceleratedbytheadditionof H2O2.However,areversetrendinthereactionratewasobserved upon the addition of excessive amounts H2O2. This was most probablyduetopoisoningofthephotocatalystsurface.Besides,the surplusamountofH₂O₂wouldalsotrapOHradicalsandtransform themtoweakerHO2

radicals whichrestrainedthedegradation performance[49].

DespiteoftheirimportantroleinPODS,ithasbeenrealizedthat the introduction of O2 or H2O2 as exterior oxidants would significantlyleadtotherisk ofexplosionoffuels,whichshould bestronglyhindered[160].Forthatreason,itissensibletoexpect thatthedevelopmentofpotential photocatalystswhich possess plentiful self-promotedelectron–hole pairs to form superoxide anionswithoutadditionofoxidizingagentismoreencouragingfor PODS[61].

Extractant

Ithasbeenreportedthatagoodextractantorsolventmusthave goodextractivecapabilityforsulphurcompoundsandbefreeof contaminantstothefuels,non-toxic,environmentallybenign,and stabileforrepetitiveuse[161].Thus,thetypeofextractantsisone oftheimportantfactorsforsulphurremovalinordertoachievea betterperformanceinPODS.Sofar,conventionalsolventssuchas

water,acetonitrile,dimethylformamide,andmethanolhavebeen exploredforthatpurpose[86,30,11,162].However,theirlimitations intermofenvironmental issuesand reusabilityhaveledtothe searchfornewpotentialextractionagents[15,16].

Theutilizationofionicliquids(ILs)asagreenreactionmedium for EDS has become the main focus, due to their fascinating properties which include thermal and chemical stability, the abilitytomaintainaliquidstateoverawiderangeoftemperatures, andtheiralmostinsignificantvaporpressurewhichisrelatedto theirioniccharacter[163].Furthermore,a careful choiceof the constituent ions canalter theirproperties, thus permittingthe designofaparticularionicliquidtoencountertheparticulartarget requirements[164].

InspiredbyILs,lowcostextractantsknownasdeepeutectic solvents(DESs),ILsderivatives,wereestablished.Thesesolvents consistof a hydrogen-bond acceptor(HBA)and hydrogen-bond donors(HBD).TheyarenotonlyadvantageouslikeILs,butalso beneficialin terms of low-cost and green synthesis procedures [165].Inthatstudy,asimplesystemconsistingofextractionand photooxidativedesulphurizationusingair,isobutylaldehyde(IBA), andDESwas developedtoachieveaprofoundsulphurremoval.

BasedonFig.6,itcouldbeobservedthatalltheabovementioned parametersarevitaltoattainaverydeepdesulphurization(96.3%).

An absence of any of those factors would result in poor desulphurization. The PODS only achieved 63.4% without the extractant, further proving its important contribution towards betterperformance.

Table4

SummaryoftheresultsforthecouplingoftwosemiconductorsinPODS.

Photocatalyst Coupling photocatalyst

Synthesismethod Lightsource Experimentalconditions Photocatalytic

performance Ref.

CeO2 g-C3N4 Electrostatic-inducedself- assemblytechnique

300Wxenonlamp(visiblelight) m(Catal)/m(DBT)=1:20 [DBT]=200mgL¹

98%in180min [101]

TiO2 g-C3N4 Two-stepprocess 250WhighpressureHglamp(UVlight) Catalyst=10.0gL¹ [DBT]=500mgL¹

98.9%in120min [147]

CoAl–MMO BiVO4 Hydrothermal 500WXearclamp(visiblelight) Catalyst=1.0gL¹ [Thiophene]=200mgL¹

98.6% [112]

CQDs ATP Impregnation 300Wxenonlamp(visiblelight) Catalyst=10.0gL¹

[DBT]=200mgL¹

93%in300min [148]

BiVO4 g-C3N4 Hydrothermal Abuilt-inxenonlamp(visiblelight) Catalyst=nodata [DBT]=100mgL¹

99%in300min [70]

MPcs La0.8Ce0.2NiO3 Sol–gelandimmersion method

UVgermicidalandiodine-tungstenlamp (simulatedsunlight)

Catalyst=3.0gL¹ [DBT]=800mgL¹

88.6%in300min [153]

Fig.6.Sulphurremovalofdifferentphotochemicaldesulphurizationsystemsin modeloil[165].

Adsorbent

Itwaspreviouslyreportedthattheadsorptioncapacityofan adsorbentforasubstancedependsonthesurfacecharacteristicsof theadsorbentas wellasthenatureof thesubstance[166].For example,toadsorb4,6-DMDBTmoleculeswithestimatedsizeof 0.59x0.89nm, an adsorbent with pores smaller than 1nm is required[4].Apartfromthat,itisalsocrucialtofindhighsurface areamaterialswhichpossesshighadsorptioncapabilitytoachieve adeepdesulphurization.Sofar,variousnoveladsorbentmaterials havebeendevelopedtoadsorbsulphur-containingcompoundsin fuel including MCM-22, Y-zeolites, heteroatom zeolites, nano- crystallineNaY, La/MCM-41,metalhalides/MCM-41and SBA-15, Cu/ZrO2,CuO/SBA-15,Ni/AlMCM-41,CeMCM-41,andvarioustypes ofZrO2-basedadsorbents[7].

Inphotocatalytic-adsorptivedesulphurization,thefirststepis thephotocatalyticoxidationofsulphurcompoundsoveracatalyst oradsorbenttoformpolarsulfoxidesorsulfones[166].Then,these higherpolarityproductsarespecificallyadsorbedonthecatalystor adsorbent.Lietal.proposedaH2O2-assistedhydrothermalmethod of TiO2–SiO2 (TiO2–SiO2–H) for improving its performance in photocatalytic-adsorptivedesulphurizationofmodeloil[167].In that study, the photocatalytic activity of TiO2–SiO2–H was compared with TiO2–SiO2, which was prepared in the absence ofH2O2.ItwasfoundthattheDBTadsorptionequilibriumonboth catalysts was achieved within 1h before UV irradiation. The equilibriumadsorptioncapacityofTiO2–SiO2–Hwashigherthan thatofTiO2–SiO2,possiblyduetoitslargersurfacearea.UponUV illumination,thesulphurremovalobtainedbyTiO2–SiO2–Hwas muchhigherthanTiO2-SiO2,whichcouldbeascribedtothelarger surfacearea,muchhigherUVlightharvestingability,andbetter chargecarrierseparation.TheSEMimagesshowedthattheTiO2– SiO2–HthatissynthesizedinthepresenceofH2O2presentswell- dispersedsphericalparticlesandsomefloccule-likenanoparticles whicharedissimilartoTiO2–SiO2.Itwas recognizedthatin the existenceofexcesswater, tetrabutyl titanate(TNBT)was easily hydrolyzedtoproduce a hydroxideprecipitate. However,when TNBT was added into the isopropanol and 30% aqueous H2O2

mixtures,theabsenceofprecipitatesignifiedtheroleofH₂O₂in slowingdownthehydrolysisrateofTNBT,mostpossiblyduetoits stronginteraction withTi species. This led tothe floccule-like nanoparticleformationthatcontributedtoahighersurfaceareaof TiO2–SiO2–HthanTiO2–SiO2forahigherUVabsorptionabilityand abetterseparationofelectron–holepairs.

Others

Theamountofphotocatalystwasanotherimportantfactorin PODSofsulphurcompoundsunderlightirradiation[67].Itiswell- knownthattheincreasingcatalystdosagewillprovidemoreactive sites for adsorption of sulphur compounds and photons, thus enhancing the number of photogenerated electron–hole pairs.

Meanwhile,increasingthedosagebeyondtheoptimumconcen- tration might lead tothe aggregation of catalyst particles and reducethesurfacearea,thusloweringphotocatalyticperformance.

Moreover, the aggregated particles might also hinder light penetration,thereforepreventingtheformationofphotogenerated electron–holepairs[49].

Besides, the different initial concentration of sulphur com- poundswasalsoasignificantfactortobeinvestigatedinPODS.It wasobservedthatphotocatalyticperformanceismorefavorableat lowsulphurcompoundconcentrationsinmostofthePODSstudies [49,168].Itisnoteworthythatbyincreasingtheinitialconcentra- tion,thePODSefficiencywasdecreased.Thiswasmostprobably due to saturation of the sulphur compound on the catalyst’s surface,aswellasinhibitionoflightpenetrationtothesurfaceof the catalyst. Therefore, the formation of the superoxide and hydroxyl radicals as important active species in PODS was

retarded, thus decreasing the performance. To date, a similar observationwasreportedinphotocatalyticdegradationofother organiccompounds suchas p-chloroaniline, bisphenol A (BPA), ibuprofen,andparacetamol[169–171,82].

Photocatalyticdesulphurizationmechanism

Generally, thephotocatalyticreactioncanbeelaboratedasa process of generation,transfer, and consumption of thephoto- generatedcarriers,whichareelectron(e)andhole(h⁺)[172].The firststepistheabsorptionofincidentphotonswithenergyabove orequaltothebandgapofsemiconductor,excitingtheelectrons fromtheVBtotheCB,andformingpositiveholesintheVB(Fig.7).

Theproducedelectronswouldreducetheabsorbedoxygentoform superoxideradicals(O²)whichcanfurtherdisproportionateto form OH via chain reactions. Meanwhile, the holes abstract electronsfromabsorbedpollutantsorreactwithH2OtoformOH [173].

To date, a major challengefor the developed photocatalysts includesfastphotogeneratedelectron–holepairsrecombination, whichisduetothestrongCoulombicforcebetweenelectronand hole (Fig. 7) [174]. Moreover, the lack of good compatibility betweenthestrongredoxabilityand lightresponserangefora single-componentphotocatalystalsolimitsitsapplication.Awide bandgapcatalystisrequiredtoachievesufficientredoxcapability foraspecificreactionoccurringonasinglephotocatalyst,whilea narrowbandgapisvitaltoenhancetheutilizationproficiencyof solarenergy.Therefore,considerableactionisneededtodevelopa facile approach to overcome these challenges for improved photocatalyticperformance.

In order to predict the PODS mechanism, two common techniqueshavebeenexploredsofar;theseareelectronscanning resonance(ESR)measurementsandscavengerexperiments.Inthe PODSprocessoverAg–TiO2dopedonporousglass(Ag-TiO2/PG) catalyst, ESR analyses were carried out using 5,5-dimethyl-1- pyrroline-N-oxide (DMPO) quencher to detect the presence of radicalsinthePODS[61].AsdepictedinFig.8aandb,allofthe analyses were conducted in the dark and under visible light illumination.Itwasobservedthatnoradicalwasperceivedinthe dark,whilefourdistinctivepeaksofO2andOHweredetected uponvisiblelightillumination,signifyingthatbothspeciesexistin

Fig.7.Generalphotogenerationmechanismofelectron–holepair.

the system. Interestingly, the active radicals produced using Ag–TiO2/PGareexactlytwofoldhigherthanthatofTiO2/PG.Fig.8c shows the determination of the photogenerated holes using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). It is noteworthy thattherewerenoholesignalswereobservedindark,whileits intensityinAg–TiO2/PGwasexpressivelyhigherthanTiO2/PG.Itis stronglybelievedthatthehighconcentrationofreactivespeciesin Ag–TiO2/PGphotocatalystsisduetothesymbioticinteractionofAg andTiO2 nanoparticlesforefficient lightharvestingand charge carrierseparation.

AscavengerexperimentproposedforPODSinvolvestheuseof differentscavengeragentstocapturetheexistingreactivespecies.

InthePODSofDBTusingBiP_1xVxO4/ATP,thePODSrateofmodel oildeclinesto24.4%,56.9%and84.9%correspondinglybyadding benzoquinone(BQ),triethanolamine(TEOA),andtert-butylalco- hol(TBA)duringthereactionexperiment.Theseresultssuggested that O₂– played the main role, assisted by h⁺, while the OH radicalsonlyhaveaminorcontributiontowardsthereaction[111].

InPODS,itisessentialtodeterminethepositionofVBandCBof eachphotocatalysttoclarifythereactionmechanism,basedonthe followingequation;

EVB¼XE^eþ 1 2Eg

ECB¼EVBEg

ð1Þ

whereEVBandECBsignifytheVBandCBedgepotentialaccordingly, X signifies the electronegativity, E^e (about 4.5eV) is the free electronsenergyonthehydrogen,andEgisthebandgapofthe

photocatalyst.Basedonthecalculationusingtheaboveequation, the VBand CB positionof BiPO4 was foundto be4.21eV and 0.05eV, respectively, and 2.13eV and -0.35eV, accordingly for BiVO₄. However, due to the unclear electronegativity of BiP_1xVxO4,itisdifficulttoattaintheVBandCBpositionofthat composite using Eq. (1), thus density functional theory (DFT) Fig.8.DMPOspintrappingESRspectraforDMPO–O2–(a),DMPO–OH(b)andTEMPO–h⁺(c)oftheTiO2/PGandAg–TiO2/PGmaterialswithdarkandvisiblelightirradiation [61].

Fig.9.PhotocatalyticdesulphurizationmechanismofBiP1xVxO4/ATP[111].